Electric furnace



Oct. 23, 1956 T. M. BUCK ET AL ELECTRIC FURNACE Filed May 28, 1955 ww@ Q United States Patent ELECTRIC FURNACE Thomas M. Buck, Plainfield, and Eric A. Hitchcock, Metuchen, N. J., assignors to National Lead Company, New York, N. Y., a corporation of New Jersey Application May 28, 1953, Serial No. 358,084

3 Claims. (Cl. 219-36) The present invention relates in general to electric furnaces of the resistor type and, in particular, to an electric furnace having a tubular resistor element which serves as a high temperature reaction chamber for the production of refractory metalloid compounds and similar products.

An object of the present invention is to provide a superior electric resistance furnace which is relatively inexpensive, embodies relatively few parts, attains high temperatures and is convenient to operate.

Another object of the invention is to provide a low voltage carbon resistance furnace which may be heated to high operating temperatures in a short time and cooled down rapidly.

A further object of the invention is to provide an electric resistance furnace of superior construction such that its resistor tube may be removed from and reassembled in the furnace with ease and dispatch.

A further object of the invention is to provide an electric resistance furnace with superior electrical contacts between an electric current source and the resistor tube such that electrical contact with the latter will be constant.

A still further object of the invention is to provide an electric furnace wherein the maximum temperature of the furnace is maintained substantially at the center of the furnace resistor tube from which point the temperature is graded down gradually to the exterior of the furnace.

These and other objects, features and advantages of the invention will be further apparent to those skilled in the art from the following description and reference to the drawing in which:

Figure l is a longitudinal sectional View through the improved electric resistance furnace of this invention.

Figure 2 is an end view of the furnace partly in section on line 2--2 of Figure 1, and

Figure 3 is an enlarged exploded perspective view of the tubular resistor element and contact tubes thereof.

The preferred embodiment of the invention is shown in the drawings and comprises, in general, a substantially cylindrical metal casing which constitutes the outer shell of the furnace; and a composite core assembly which is mounted within the metal casing and comprises, in the main, a pair of pyramidal current concentrating conductors, each consisting of an end plate of the furnace and current conducting heat resistant elements of progressively decreasing cross-sectional area including the tubular resistor element which extends between the end-plates of the furnace. The aforesaid pyramidal current concentrating conductors of the core assembly are designed to conduct electric current from a current source to the tubular resistor element and, as hereinafter described, to concentrate the current to a relatively small section of the resistor element, thereby to develop extremely high temperatures at this point in the resistor tube; and to present heat resistant surfaces of increasingly large area from this high temperature point outwardly to the respective end plates of the furnace for protecting the latter from the high temperature at the center of the furnace.

In the embodiment shown in the drawings, the metal casing of the furnace is indicated generally at 10 and comprises a substantially cylindrical jacket formed of iron, brass or other suitable metal and provided at its opposite ends with flanges 11--11 respectively, each of appreciable thickness and provided with a plurality of apertures arranged circumferentially therearound to accommodate fastening means such as the machine screws 52 which are used, as hereinafter described, to secure the respective end plates of the core assembly to the casing with an air-tight seal, The interior of the metal casing 10 constitutes a cylindrical chamber within which the tubular resistor element 12 is mounted substantially concentric to but spaced from the wall of the casing. Wrapped around the exterior of the casing 10 and the rims of its flanges 11 are a plurality of turns of copper tubing 13 which constitute a cooling coil through which a coolant such as water is circulated to dissipate heat from the furnace casing.

Turning now to the pyramidal current concentrating conductors of the furnace, these conductors are substantially identical and hence, for the sake of brevity, the description which follows will be limited to the features of one pyramidal conductor only, identical members being identified by similar figures.

Each pyramidal conductor comprises, in part, a relatively thick metal disc or end plate 14 formed preferably of brass or similar electrical conducting material and provided around its periphery with a cooling coil comprising a plurality of turns of tubing 1S soldered or otherwise secured on the rim of the end plate in a manner to augment heat transfer from the end plate to the cooling coils through which water is adapted to be circulated for removing heat from the end plate. As shown especially well in Figure l, these cooling coils 15 are continuous with additional cooling coils which are secured in a spiral arrangement on the outer face of the end plate 14, as shown especially well in Figure 2, and with cooling coils wrapped around a hereinafter described axial tube which extends outwardly from the outer face of the end plate. The inner face of each end plate is provided with an integral substantially circular concentric boss, as indicated at 16, which serves as one of the current conducting heat resistant elements of the pyramidal conductor, the diameter of the boss 16 being less than the overall diameter of the end plate 14 and less than the inside diameter of the furnace casing. Extending through each end plate is an axial aperture having a counterbore 17 intersecting the inner face of the boss 16, the diameter of the counterbore being substantially one-half the overall diameter of the boss and the depth of the counterbore being substantially twothirds the thickness of the end plate. The portion of the axial aperture which extends from the base of the counterbore 17 to the outer face of the end plate is threaded to make a screw threaded connection with the inner externally threaded end of a brass axial tube 18 which projects outwardly from the outer face of the end plate 13. The axial tube 18 is adapted to be locked in the axial aperture of the end plate 14 and to this end a lock nut 19 is provided on the externally threaded end of the tube 18 and adapted to be turned up tightly into engagement with the outer face of the end plate. The axial tube 18 has a substantially uniform internal diameter which corresponds to or is only slightly larger than the outside diameter of the tubular resistor 12, hereinafter described, and extends outwardly from the outer face of the end plate 14, the length of the axial tube 13 being conveniently about one-half the overall length of the furnace.

l The outer end of the axial tube 18, at the left-hand end' of the furnace, as seen in Figure 1, is adapted to support an observation window and to this end is provided with a tiange 20, the outer face of which constitutes a seat for a relatively thick glass window 21 which is `held against the insulated face of the flange by an annular ring 2,2 which, in turn, is removably secured to the flange 23 by means of screws n23. The tube V18 of the en d plate 14 at the right-hand end of the furnace is identical to the tube hereinabove described except that no window `is provided in the end thereof, the latter being closed by a removable cover-plate 24having an aperture therein toraccommodate a gas outlet pipe 25. A gas inlet pipe 26 projects radially from the axial tube 13 of the end plate 14 by which means an inert gas such as argon may be continually fed into o ne end of the tubular reactor and exhausted from the other end, thereby effectively excluding and/ or removing oxygen from the reactor tube. In addition to these inlet and outlet pipes, a second set of gas pipes, indicated at 27 and 2S respectively, are provided in the respective end plates 14-14 for sweeping an inert gas through the interior of the furnace to assist in absorbing heat radiated from the tubular reactor and expelling oxygen and other deleterious gases from the furnace.

Each end plate 14 of the furnace constitutes an electric conductor for conducting current to the resistor tube 12 of the furnace, and to this end each end plate is provided with a terminal post 29 which extends outwardly from the outer face thereof and is provided at its outer extremity with a nut and washer assembly for fastening the positive and negative leads 30 and 31 respectively, of a source of electric current thereto.

Among the characteristic features of the resistance furnace of this invention is the relative rapidity which the furnace may be brought up to operating temperatures and the rapidity with which the furnace cools down upon opening the circuit. These characteristics stem from the novel construction of the pyramidal current concentrating conductors of the core assembly and in particular the above described end plates 14-14, the resistor tube 12 and a pair of graphite blocks 32--32 which support opposite ends respectively of the resistor tube. lt will be noted that these elements are of progressively decreasing cross-sectional area, which construction serves not only to concentrate the current at substantially the center of the resistor tube 12 but also to facilitate dissipation of the heat from this point to the end plates. Referring to Figure l, the block 32 of each end plate 14 is a substantially cylindrical graphite element of electrode grade graphite having an axial bore 33, the diameter of which corresponds substantially to or it only slightly larger than the diameter of the resistor tube; and an annular ange 34 at its outer end adapted to seat snugly in the counterbore 17 of the corresponding end plate and to be held therein by means of a retaining ring 35 secured to the boss 16 of the end plate 14 by screws 36 or equivalent fastening means. As shown in Figure l, the thickness of the annular flange 34 is somewhat greater than the depth of the counterbore 17 such that the retaining ring 35 is spaced outwardly from the face of the boss 16, thereby providing clearance space therebetween for the admission of gas by way of the inlet pipe 27. A similar arrangement at the opposite end of the furnace permits the escape of gases from the furnace by way of the outlet pipe 28. The opposite end, sometimes hereinafter referred to as the inner end of each graphite block, is turned down or otherwise reduced in diameter and forms a shouldered portion 37 which serves to support one end of a heat insulating sleeve of the core assembly as described below.

The resistor tube 12 is shown especially well in Figure 3 and comprises a hollow graphite tube of electrode grade graphite, the overall length of which corresponds roughly to the distance between the end plates 14-14 of the furnace. The axial bore 3S of the resistor tube is of substantially uniform diameter throughout its length and constitutes a reaction chamber within which the products to be heated are placed. In this connection, the

products to be reacted may be held in a refractory boat, a Crucible, or simply wrapped in wax paper. The two outer end portions 39-39 of the resistor tube are of a diameter to make a close lit in the axial bores 33-33 of the shoulder graphite blocks 32--32 hereinabove described. The portion of the resistor tube intermediate its outer end portions 39-39 is turned down or otherwise reduced in diameter to form a relatively short center section 40 at which point the wall thickness of the tube is reduced to a minimum dimension consistent with the strengtl re 1irernents of the tube. In accordance with this construction, the electrical resistance of the reduced center section il of the resistor tube is at a maximum value, thereby providing maximum temperatures at this section of the tube.

lt will be apparent, therefore, that at points progressively remote from the center section 40 of the resistor tube, the cross-sectional areas of the current conducting elements including the enlarged outer end portions 39-39 of the resistor tube, the shoulders 37-37 of the blocks 32-32, the body portions of said blocks, the bosses 16-16 of the end plates and nally the end plates 142-14 themselves are of progressively increasing diameter. Hence, the current being fed to the resistor tube 12 by way of these elements encounters maximum resistance at the center section itl of the resistor tube i. e. is concentrated at this point and converted into maximum temperatures; while the heat developed at the center of the resistor tube will be conducted away by conducting areas of progressively increasing dimensions which provide for a gradual reduction in temperature outwardly. In accordance with this construction, the furnace may be brought up to maximum operating temperatures in a short time and upon termination of the heating cycle will cool off in a relatively short time as, for example, of the order of about 30 to 40 minutes.

Referring again to the heat insulating sleeve of the core assembly, the sleeve is indicated at 41 and comprises preferably a thin walled tube of a high heat resisting material such as molybdenum, the length of the tube 41 being such as to span the distance between the shoulders 37-37 of the graphite blocks 3.2-32 when the latter are assembled in the furnace casing as shown in Figure 1. The opposite ends respectively of the molybdenum tube d1 are supported on the shoulders 37-37 of the blocks 32-32 by means of electrical insulating rings l2-42, each insulating ring being formed of a highly refractory material such as aluminum oxide, known commercially as Alundum, or an equivalent materia-l, and having an internal diameter such as to nt closely on the shoulders of the graphite blocks. The outside diameter of each .electrical insulating ring is such as to t snugly into one end of the molybdenum tube 41. ln a preferred construction the thickness of each electrical insulating ring is about one-half inch. The heat insulating sleeve 41 is thus electrically insulated from the two graphite blocks 32-32, thereby restricting the passage of the current to the thin walled center section 40 of the resistor tube. The insulating sleeve 41 also serves to confine the heat developed in the thin walled section of the tube against radiation and/ or convection to the outer shell or casing 10 of the furnace. In this connection such heat as may pass through the molybdenum sleeve 41 is adapted to be restrained from heating the outer shell of the furnace by means of a relatively thick heat resistant porous carbon tube 43 which is telescoped over the assembled graphite blocks 32-.32, the resistor tube 12 and the molybdenum sleeve 41, the porous carbon tube 43 being supported at its opposite ends respectively by electrical insulating rings 44-44 which are mounted on the respective graphite blocks 32-32 adjacent the respective retaining rings 35-35, the electrical insulating rings 44-44 being engaged in counterbores 45-45 formed in the ends of the porous carbon tube 43. As shown in Figure 1, the diameter of the bore of the porous carbon tube 43 is such as to provide clearance around the walls of the molybdenum sleeve 41 to inhibit the transfer of heat thereto while the outside diameter of the carbon tube is such as to leave a clearance space between the outer walls thereof and the inner walls of the furnace casing for the passage of gas therethrough. In this connection the inert gas which is swept through the space between the composite core assembly and the outer casing of the furnace may remove heat from this space as Well as any reaction gases that may have leaked or otherwise escaped into this area from the reactor tube.

In order to insure constant and consistently uniform contact between the enlarged end portions 39-39 of the resistor tube and the walls of the axial bores 33-33 of the graphite blocks 32-32 which serve to conduct current to the resistor tube from the respective end plates 14-14, pressure applying means are provided at opposite ends respectively of the resistor tube to constantly urge the respective end portions 39-39 of the tube into engagement with the walls of the aforesaid bores 33--33. To this end, the opposite ends respectively of the resistor tube are formed with bevelled faces 46-46 as shown especially in Figures 1 and 3; and slidably supported in the respective axial tubes 18-18 of the respective end plates are relatively short graphite contact sleeves 47 47, the inner ends of which are formed with bevelled faces 48-48 corresponding to the bevelled faces 46-46 on the respective ends of the resistor tube. The opposite or outer ends of the respective contact sleeves 47-47 are squared off, the contact tube at the right-hand end of the furnace being engaged by a washer 49 which forms a bearing surface for the inner end of a coil spring 50, the outer end of which is engaged against the aforesaid cover-plate 24 of the axial tube 18. The outer end of the contact sleeve 47 at the left-hand end of the furnace is engaged by a short graphite collar 51 having peripheral apertures therein adapted to be disposed substantially opposite the gas inlet tube 26 to permit gas to enter the central bore 38 of the resistor tube. A washer 49 engaged against the outer end of the collar 51 serves as a seat for a coil spring 50 which is interposed between the washer 49 and the aforesaid window 21. The coil springs 50-50 thus serve to urge the respective graphite contact sleeves 47-47 inwardly axially such that the bevelled faces 48-48 of the contact sleeves exert a constant camming force against the respective bevelled faces 46-46 of the resistor tube 12, thereby camming the latter transversely against the adjacent walls ofthe axial bores 33-33 so as to maintain constant line contact therewith.

` The elements of the furnace are assembled by first preassembling the flanged ends of the respective graphite blocks 32-32 in the counterbores 17-17 of the end plates 14-14, thereafter securing one end plate 14 to the flange at one end of the casing 10 by means of the machine screws 52 which are electrically insulated from the end plate so as to prevent the jacket or casing 10 from short circuiting the end plates. One end of the molybdenum heat insulating sleeve 41 is then telescoped over an electrical insulating ring 42 on the reduced shoulder portion 37 of one of the graphite blocks, and the corresponding end of the porous carbon tube 43 is telescoped over the electrical insulating ring 44 on the other end of the corresponding block 32. The second end plate 14 is then secured to its respective end of the jacket 10 while simultaneously telescoping the corresponding ends of the molybdenum sleeve and carbon tube 43 over the electrical insulating supporting rings 42 and 44 respectively. After assembling the end plates on the jacket 10, one end of the resistor tube 12 is inserted into the open end of one of the axial tubes 18, pushed through the axial bore 33 of the corresponding graphite block and into the axially aligned bore of the opposite graphite block such that the reduced center section 40 of the tube 12 is substantially intermediate opposite ends of the furnace. The respective contact sleeves 47-47 are inserted into the respective axial tubes and oriented so that their bevelled faces 48-48 are in contact with the corresponding bevelled faces 46-46 of the resistor tube. The washer and spring assemblies, including the graphite collar 51, are then assembled in the outer ends of the respective axial tubes and thereafter the window 21 is secured on the outer end of its respective axial tube by the ring 22 and screws 23. The cover plate 24 is left off until a charge has been put into the resistor tube and the furnace is ready to operate. Flexible connections are then made to the respective gas inlet and outlet pipes and to the inlet and outlet pipes of the cooling coils.

The operation of the furnace may be described briey as follows: A boat, crucible, or the like, containing the products to be reacted, is charged into the open l end of the resistor tube and moved to the center section 40 thereof after which the cover plate 24 is secured in place. Although the charge to be heated may be carried in a crucible or the like, the resistor tube itself may be charged with the materials to be heated and either during or after assembly of the resistor tube in the furnace. inert gas such as argon is fed into the inlet pipes of the resistor tube and furnace respectively for sweeping the resistor tube and the interior of the jacket free of oxygen. Water or a similar coolant is also delivered to the cooling coils of the end plates 14-14 and of the jacket 10 and finally the leads 30 and 31 of the source of electric current are connected to the terminal posts 29-29 of the end plates whereby electric current is conducted to the resistor tube and in particular to the center section 40 thereof which is heated by the current to temperatures of from 2000 to about 2500 C. The reaction products are thus heated at a high ternperature and for a sucient length of time to complete the reaction and thereafter the current to the leads 30--31 is cut off and the furnace permitted to cool down. During the heating cycle the circulation of water through the cooling coils of the end plates and the jacket 10 maintains the jacket relatively cool, and after cutting off the current to the furnace, the dissipation of the residual heat is enhanced by means of the pyramidal conductors and the circulation of water through the cooling coils so that in a relatively short time the furnace is sufficiently cool to permit removal of the reacted products.

It will thus be seen that the furnace of this invention is characterized by its simple and inexpensive construction, by the ease with which the furnace may be charged, and by an effective concentration of heat such that maximum temperatures are achieved rapidly and maintained in a relatively limited region within the interior of the furnace; and thereafter heat is removed from the furnace in a relatively short time. The furnace is also characterized by pressure loaded contact members which by camming engagement with the tubular resistor hold the latter in constant and uniform contact with the current conducting means.

While this invention has been described and illustrated by the drawings shown, it is not intended to be strictly limited thereto, and other variations and modifications may be employed within the scope of the following claims.

We claim:

l. An electric furnace comprising an outer casing; current conductors at opposite ends respectively of said casing, said current conductors comprising centrally apertured metal end-plates, each end-plate being arranged to seal the corresponding end of said casing and having a massive graphite block assembled on the inner face thereof and projecting inwardly into the interior of said casing, said graphite blocks having axially aligned bores in alignment with the central apertures of said end-plates, and outer and inner concentric shoulders of progressively decreasing diameter; heat insulating rings mounted on the respective shoulderslof. said graphiteA blocksga reaction chamber in said.. housing' comprisings a graphite tube having: a,V relatively'I thin. Walled center section` and. enlargedendportions, the, latter being. arrangedrto engage in the bores. of' the respective. graphite blocks; av heat insulating'sleeve; surrounding the thin walled center section of said graphite tube, saidheatV insulating sleeve being-supported'rat its.oppositel ends respectively by the heatinsulating rin-gs mounted onthe correspondingr inner shoulders of said'graphite blocks; a second heat insulatn ingfsleeve concentric with said rst insulating sleeve and supported-at itsopposite ends respectivelyv on the insulating rings mounted on the corresponding outer shoulders of said graphite blocks; and terminals connected to said end-plates and arranged to transmit electric current from av current source through said graphite blocks to said graphite tube.

2. An electric furnace comprising an outer casing; current'conductors at oppositeends respectively of said casing, said currentV conductors comprising centrally ap ertured metal end-plates, each end-'plate being arranged' to sealthe correspondingrendgofsaidcasing and'having a massive graphite block assembled in the inner face thereof and projecting inwardly into the interior of said casing, said graphite blocks having axially aligned bores in alignment with the-central apertures of said end-plates, and outer andinner concentric shoulders 'of progressively decreasing diameter; heat insulating rings mounted on the respective shoulders of said graphite blocks; a reaction chamber in said housing. comprising a graphite tube having arelatively thin walledtcenter section and enlarged end portions, thelatter'being'arranged to engage in the bores ofV theV respective graphite blocks; cam facesion opposite ends respectively of'said. graphite tube; camming means comprising' spring loaded electrical conducting sleevesfassembled inthe boresof the respective graphite blocks andprovided withrbevelled'faces adapted to carnmingly engage the bevelled faces on the corresponding endsv of thegraphite tube to cam the enlarged end portions thereof into constantV contact Withthe Walls of the bores of said graphite blocks; a heat insulating sleeve surroundf ing the thin walled center sectionof said graphite tube, said lheat insulating sleevebeing supported at its opposite ends respectively byvtheheatinsulating rings mountedvon thefcorresponding'inner'shoulders of said graphite blocks;- a second heat insulating sleeve concentric with said first insulating sleeve and supportedv at its opposite ends respectivelyV on theinsulating ringsmounted on the corresponding outer shoulders of said graphite blocks; and terminals ,connected'to said end-plates and arranged. toY transmit electric current from a current source through` said-graphiteblocks to said graphiteY tube.

3.- An electric furnace comprising an outer casing; current conductors at opposite ends respectively of said casingsaid current conductors comprising centrallyapertured metal end-plates,` each" endplate beingV arrangedl to seal the corresponding. end of said casingand having at massive graphite block assembled: on the inner,` face thereof and projecting inwardly into the interior of said casing, said graphite blocks havingv axially aligned boresl in alignment withsthe Ycentralapertures of said end-plates.

and outer and inner concentric shoulders of progressively decreasing diameter; heat insulating rings mountedon the respective shoulders of said graphite blocks; a reaction chamber in said housing comprising a graphite tube'having` a. relatively thin Walled center section and enlarged end portions, the latter being arranged to engage in theV bores.

of the. respective graphite blocks; cam faces on opposite ends respectively of said graphite tube; camming means comprising spring. loaded electrical conducting sleeves assembled in the bores of the respective graphite blocks and provided with bevelled faces adapted to carnrningly engage the bevelled faces on thecorrespondingk ends of the graphite tube. to cam the enlarged` end portions thereof into constant contact'withthe Walls of the boresof said graphite blocks; athin walled heat insulating; sleeve surrounding the thin Walled center section of said graphite tube, saidthin walled heat insulating sleeverbeing supported at.its.opposite ends respectively by the heat insulating, rings-1 mounted` on the. corresponding; inner shoulders of said graphite blocks; a relatively thick walledheat insulating sleeveV concentric Withsaid thin walled heat insulatingsleeve and intermediate said thin walled heat insulating sleeve andthe Wall of said' casing, saidthick Walledheat insulating sleeve being supported atits opposite endszrespectively on the insulating rings mountedv on the corresponding outer shouldersof said graphite blocks; and terminals connected to said end-platesandarranged to transmit electric current from a current source through-said graphite blocks to said graphite tube.

References Cited in the leof this patent UNITED Y STATES PATENTS 

